Stainless Steel Seamless Coil Tube for Economizer & Low-Pressure Heater Systems (304TP / 316TP)
Product Overview
The Stainless Steel Seamless Coil Tube (Coiltube) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered tubular product specifically designed for critical heat-transfer applications within thermal power generation systems. These coil tubes are deployed in economizers (省煤器) and low-pressure heaters (低压加热器), where they serve as the primary conduits for feedwater circulation under elevated temperature and pressure conditions.
Operating within a feedwater temperature range of 150 °C to 300 °C, these coil tubes are subjected to rigorous chemical and mechanical environments. A central design consideration is the strict oxygen content control within the feedwater system, as dissolved oxygen is a primary driver of oxidative degradation in austenitic stainless steel. Additionally, the product is engineered with particular attention to Flow-Accelerated Corrosion (FAC), a well-documented failure mechanism in high-velocity water circuits typical of economizer and low-pressure heater installations.
Material & Properties
Available Grades
- 304TP (UNS S30403 / EN 1.4301): The "TP" suffix denotes a high-purity, thermally treated variant of the standard 304 austenitic stainless steel. It features reduced interstitial carbon (≤ 0.03% C), lower sulfur content, and superior resistance to sensitization during welding and heat exposure. This grade offers excellent general corrosion resistance, good formability, and reliable performance in moderate chloride environments.
- 316TP (UNS S31603 / EN 1.4401): A molybdenum-enhanced variant of 304TP, containing 2.0–2.5% Mo. The molybdenum addition significantly improves resistance to pitting and crevice corrosion, making 316TP the preferred choice where feedwater chemistry may exhibit localized corrosive conditions or elevated chloride concentrations.
Mechanical Properties (Typical — Annealed Condition)
| Property | 304TP | 316TP | Test Standard |
|---|---|---|---|
| Tensile Strength (Rm) | ≥ 520 MPa | ≥ 515 MPa | ASTM A270 / EN 10216-5 |
| Yield Strength (Rp0.2) | ≥ 205 MPa | ≥ 200 MPa | ASTM A270 / EN 10216-5 |
| Elongation (A50) | ≥ 40 % | ≥ 40 % | ASTM A270 |
| Hardness (HBW) | ≤ 210 | ≤ 210 | ASTM A262 |
| Maximum Service Temperature | 870 °C | 870 °C | Material Data Sheet |
| Thermal Conductivity (25 °C) | 16.3 W/(m·K) | 16.3 W/(m·K) | ASTM E1461 |
Chemical Composition (wt. %, Maximum Unless Noted)
| Element | 304TP | 316TP |
|---|---|---|
| Carbon (C) | ≤ 0.030 | ≤ 0.030 |
| Manganese (Mn) | ≤ 2.00 | ≤ 2.00 |
| Phosphorus (P) | ≤ 0.045 | ≤ 0.045 |
| Sulfur (S) | ≤ 0.030 | ≤ 0.030 |
| Chromium (Cr) | 18.00 – 20.00 | 18.00 – 20.00 |
| Nickel (Ni) | 8.00 – 10.50 | 10.00 – 14.00 |
| Molybdenum (Mo) | — | 2.00 – 2.50 |
| Nitrogen (N) | ≤ 0.10 | ≤ 0.10 |
Specifications
| Parameter | Specification | Standard Reference |
|---|---|---|
| Product Type | Seamless Stainless Steel Coil Tube (Coiltube) | ASTM A269 / ASTM A270 / EN 10217-7 |
| Outer Diameter (OD) | 8.0 mm – 60.0 mm (custom ranges available) | Per customer specification |
| Wall Thickness | 0.5 mm – 3.0 mm (custom ranges available) | Per customer specification |
| Coil Diameter (Bundle OD) | 200 mm – 800 mm (standard); up to 1200 mm on request | EN 10217-7 |
| Surface Finish | 2B (cold-rolled and annealed) or BA (Bright Annealed) | ASTM A480 / ASTM A666 |
| End Condition | Plain end, beveled end, or threaded (per application) | ASME B31.3 |
| Feedwater Temperature Range | 150 °C – 300 °C | Plant design specification |
| Operating Pressure (Typical) | Up to 20 MPa (design-dependent) | ASME Section I / VIII |
| Design Life | ≥ 25 years (with proper water chemistry management) | Industry practice |
| Heat Treatment | Solution annealing at 1050 °C ± 25 °C, followed by water quench | ASTM A269 |
Manufacturing Process
1. Raw Material Selection
High-purity stainless steel billets or round bars conforming to ASTM A269/A270 and EN 10216-5 are procured from certified suppliers. Incoming material undergoes spectroscopic chemical analysis (OES — Optical Emission Spectroscopy) and mechanical property verification to ensure compliance with the specified TP-grade chemistry.
2. Hot Piercing & Extrusion
Billets are heated to 1150–1250 °C in induction or gas-fired furnaces and pierced using a rotary piercing mill or hot extrusion press to produce seamless tube blanks. The extrusion route offers superior grain structure control and is preferred for the TP grades to minimize inclusion alignment.
3. Cold Drawing / Rolling
Hot-finished tube blanks are cold-drawn through a series of precision dies (typically 3–5 passes) or cold-rolled to achieve the target outer diameter and wall thickness with tight dimensional tolerances (± 0.05 mm). This step develops the required mechanical strength and surface finish while maintaining the austenitic microstructure.
4. Intermediate Annealing
Where cold reduction exceeds 60%, intermediate solution annealing at 1050 °C ± 25 °C with rapid water quenching is performed to relieve work-hardening and restore ductility for subsequent forming operations.
5. Coiling & Heat Treatment
Straight tubes are formed into coils using CNC-controlled bending equipment with tooling radii calculated to prevent ovality, wrinkling, or wall thinning. Final solution annealing is conducted in a controlled-atmosphere furnace (protective gas or vacuum) to eliminate residual stresses, ensure full austenitic phase stability, and achieve the required surface condition (2B or BA).
6. Surface Finishing
Coils undergo passivation treatment (nitric acid or citric acid based) to remove free iron and establish a stable chromium oxide passive film. For BA finish, a final bright anneal in a hydrogen or nitrogen atmosphere furnace is performed, followed by electropolishing or mechanical polishing where required.
7. Quality Inspection
Every coil undergoes a comprehensive inspection protocol including:
- Visual Inspection: Surface quality verification per ASTM A666
- Dimensional Inspection: OD, wall thickness, coil diameter, and straightness measurement
- Hydrostatic / Pneumatic Testing: Pressure testing at 1.5× working pressure per ASTM A270
- Non-Destructive Testing (NDT): Eddy current testing (ASTM E309) for internal and external defect detection; ultrasonic testing (UT) for wall thickness verification
- Intergranular Corrosion Testing: ASTM A262 Practice E (oxalic acid) to confirm resistance to sensitization — critical for TP grades
- Hardness Testing: Vickers hardness (HV) verification to confirm annealed condition
Applications
Economizer (省煤器) Systems
In thermal power plants, the economizer is a heat-exchange component located downstream of the boiler furnace and upstream of the boiler proper. Feedwater enters the economizer at approximately 150–250 °C and is preheated by flue gas before entering the boiler drum. Stainless steel coil tubes in this application must withstand:
- Thermal cycling between feedwater inlet and outlet temperatures
- High-velocity feedwater flow (typically 2–4 m/s) that can induce Flow-Accelerated Corrosion (FAC)
- Variable dissolved oxygen levels that must be maintained below 7 ppb (0.1 µg/L) in deaerated feedwater systems per ASME PTC 43
- Long-term exposure to high-purity water at elevated temperatures
Low-Pressure Heater (低压加热器) Systems
Low-pressure heaters (also known as low-pressure feedwater heaters or LP heaters) are surface-type heat exchangers in the feedwater heating train, typically operating at feedwater temperatures of 100–200 °C with extraction steam from the low-pressure turbine stages. Stainless steel coil tubes in this application face:
- Condensation and flashing on the steam side
- Continuous immersion in deaerated feedwater on the water side
- Thermal expansion differentials between tube and shell
- Corrosion risk at tube-to-tubesheet joints due to crevice formation
Flow-Accelerated Corrosion (FAC) Mitigation
Critical Design Consideration: FAC is a localized corrosion mechanism that occurs when high-velocity water flow (typically > 2 m/s) over a metal surface exceeds the rate at which the protective oxide film can regenerate. In economizer and LP heater applications, FAC can lead to significant wall thinning, micro-pitting, and ultimately tube rupture. Wenzhou Jixing's 304TP and 316TP coil tubes are selected and manufactured with the following FAC mitigation strategies:
- High-purity chemistry (TP grade): Reduced carbon and sulfur content minimizes carbide precipitation at grain boundaries, preserving the integrity of the passive film under flow stress.
- Molybdenum enhancement (316TP): The Mo addition stabilizes the passive oxide layer, increasing resistance to localized breakdown under high-velocity flow conditions.
- Superior surface finish: BA or electropolished surfaces reduce the number of nucleation sites for FAC initiation.
- Optimized coil geometry: Coil design minimizes internal flow separation zones and stagnant areas where oxygen concentration cells can develop.
Quality Certifications & Standards Compliance
| Category | Standard / Certification | Description |
|---|---|---|
| Product Standard | ASTM A269 / ASTM A270 | Austenitic stainless steel seamless tube for general and high-temperature service |
| European Standard | EN 10216-5 / EN 10217-7 | Seamless tubes for heat exchangers and pressure equipment |
| Chinese National Standard | GB/T 14976 / GB/T 21830 | Stainless steel seamless tubes for pressure equipment |
| Material Certification | EN 10204 3.1 / 3.2 | Inspection certificate with third-party testing (3.2) |
| Quality Management | ISO 9001:2015 | Quality management system certification |
| Environmental Management | ISO 14001:2015 | Environmental management system certification |
| Nuclear Qualification (Optional) | ASME NQA-1 | Nuclear Quality Assurance for qualified facilities |
| Pressure Vessel Compliance | ASME Section I / VIII Div. 1 | Power boiler and pressure vessel code compliance |
Unique Selling Points
- TP-Grade Chemistry: The use of high-purity, low-carbon (≤ 0.0